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280:. This reduces the effect of wave drag without significant negative effects except at very low speeds. In the case of fighters, this was a concern, especially at landing, but in the case of helicopters, this is less of an issue because the rotor tips do not slow significantly, even during landing. Such swept-tips can be seen a number of helicopter types from the 1970s and 80s, notably the
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The BERP blade employs a final geometry that performs as a swept tip at high Mach numbers and low angles of attack, yet also enables the tip to operate at very high angles of attack without stalling. This latter attribute was obtained by radically increasing the sweep of the outermost part of the tip
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Helicopters have the additional problem that their rotors move in relation to the fuselage as they rotate. Even when hovering, the rotor tips may be travelling at a significant fraction of the speed of sound. As the helicopter accelerates, its overall speed is added to that of the tips, meaning that
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BERP IV uses: a new aerofoil, revised blade tip shape, and increased blade twist. After 29 hours of testing it has been found to, "improve rotor flight-envelope performance, reduce power needs in hover and forward flight, ... decrease airframe and engine vibration for a range of take-off weights."
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The methodology used in the design of the BERP blade ensures that the effective Mach number normal to the blade remains nominally constant over the swept region. The maximum sweep employed on the large part of the BERP blade is 30 degrees and the tip starts at a non-dimensional radius r/R=cos 30 =
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Evidence has shown that a strong "notch" vortex is also formed, which is trailed streamwise across the blade. This vortex acts like an aerodynamic fence and retards the flow separation region from encroaching into the tip region. Further increases in angle of attack make little change to the flow
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helicopter. In 1986, a Lynx specially modified registered G-LYNX set an absolute speed record for helicopters over a 15 and 25 km course by reaching 400.87 km/h (249.09 mph). Following the successful technology demonstration, the BERP III blade went into production.
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speed. The point where this occurs can be improved by making the rotor spin faster, but then it faces the additional problem that at high speeds the forward-moving blades are approaching the speed of sound and begin to suffer from wave drag and other negative effects.
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structure until a very high angle of attack is reached (in the vicinity of 22 degrees!) when the flow will grossly separate. For a conventional tip planform, a similar gross flow breakdown would be expected to occur at about 12 degrees local angle of attack.
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Therefore, the BERP blade manages to make the best of both worlds by reducing compressibility effects on the advancing blade and delaying the onset of retreating blade stall. The net result is a significant increase in the operational flight envelope.
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movements aft of the blade elastic axis (which can introduce undesirable aerodynamic and inertial couplings) are not experienced, then the tip must be configured with an area shift forward. This can be kept to a minimum by recognizing that the
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BERP III designs have a notch toward the outer end of the rotor blade, with a greater amount of sweepback from the notch to the end of the blade compared to inboard of the notch. BERP III culminated in a technology demonstration on a
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86% radius. The area distribution of this tip region is configured to ensure that the mean tip centre of pressure is located on the elastic axis of the blade. This is done by offsetting the location of the local 1/4-
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accelerates the air. These shock waves radiate away a great amount of energy that has to be supplied by the engines, which appears to the aircraft as a whole as a large amount of additional drag, known as
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This offset also produces a discontinuity in the leading edge (referred to as a notch), which results in other interesting effects. For example, recent calculations using a CFD code based on the
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It is the ability of the rotor to change its lift pattern that puts a limit on the forward speed of a helicopter; at some point the forward speed means the rearward-moving blades are below their
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geometry into the more moderately swept region. At a sufficiently high angle of attack, the vortex will initiate close to the forward most part of the leading edge near the "notch" region.
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form in areas where the local flow is accelerated above the speed of sound. This normally occurs on curved areas, like cockpit windows, leading edges of the wing, and similar areas where
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corresponding to the retreating side of the disk. In fact, experience has shown that a swept tip blade can have an inferior stalling characteristic compared to the standard blade tip.
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structure which rolls around the leading edge and eventually sits over the upper surface (as on a delta wing aircraft). This mechanism is enhanced by making the leading edge of the
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As the angle of attack is increased, then this vortex begins to develop from a point further and further forward along the leading edge, following the
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used on UK navy Sea Kings. Under test it was found to last five times longer, 195 minutes vs 39 min. The programme ended in August 2007
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Brocklehurst, Alan. AIAA-1990-3008, "Experimental and numerical study of the
British Experimental Rotor Programme blade". AIAA, 1990.
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is varying along the blade so we do not have to use a constant sweep angle, thereby minimizing the amount of forward area shift.
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as a co-patentee. The goal was to increase the helicopters lifting-capability and maximum speed using new designs and materials.
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We must also recognize that a swept tip geometry of this sort will not necessarily improve the performance of the blade at high
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the blades on the forward-moving side of the rotor sees significantly higher airspeed than the rearward-moving side, causing a
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Additionally "Rotor hub loading has been found to be the same or less than with the BERP III blade now fitted to the
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of the blades to ensure the lift is similar on both sides, in spite of the great differences in relative airflow.
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One solution to the problem of wave drag is the same that was seen on 1950s jet fighters, the use of
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Because the leading edge is so highly swept, this leading edge separation develops into a
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rotor blades. This resulted in producing new main rotor and tail rotor blades for the
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The initial programme, BERP I, studied the design, manufacture and qualification of
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525:" American Helicopter Society 64th Annual Forum, April 29 – May 1, 2008
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sections for future rotor blades. This fed into the BERP III programme.
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BERP IV The Design, Development, and
Testing of an Advanced Rotor Blade
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ENAE 632 - The
British Experimental Rotor Program (BERP) Blade
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The
British Experimental Rotor Program (BERP) Blade
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90:Tokyo Metropolitan Police Department EH101 (AW101)
581:Air Vectors: The Westland Scout, Wasp, & Lynx
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43:but its sources remain unclear because it lacks
291:However, to ensure that centre of gravity or
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156:. Unsourced material may be challenged and
220:Learn how and when to remove this message
74:Learn how and when to remove this message
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491:
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98:blade design was developed under the
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542:"BERP IV gives Merlins more payload"
509:University of Maryland, College Park
154:adding citations to reliable sources
100:British Experimental Rotor Programme
623:Science and technology in Somerset
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337:in this region relatively sharp.
594:article on the Lynx's BERP rotor
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397:Applications of BERP technology
261:. This requires changes in the
540:Coppinger, Rob (22 May 2007).
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586:"Fastest Blades in the World"
521:Harrison, Stacey, Hansford "
308:axis forward at 86% radius.
108:Royal Aircraft Establishment
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462:"Martin Lowson (Obituary)"
401:Current applications are:
512:, Retrieved 11 April 2010
474:. London. 12 August 2013
232:As objects approach the
29:This article includes a
313:Navier-Stokes equations
58:more precise citations.
110:(RAE), with Professor
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613:Helicopter components
242:Bernoulli's principle
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628:Westland Helicopters
591:Flight International
495:J. Gordon Leishman "
429:AgustaWestland AW101
411:AgustaWestland AW101
150:improve this section
104:Westland Helicopters
588:, 27 December 1986
417:Westland Super Lynx
259:dissymmetry of lift
618:Helicopter history
608:British inventions
544:. flightglobal.com
502:2007-08-21 at the
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148:Please help
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118:How it works
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50:Please help
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548:27 November
478:27 November
298:Mach number
238:shock waves
56:introducing
602:Categories
448:References
356:Programmes
278:wing sweep
180:newspapers
96:BERP rotor
64:April 2009
471:The Times
441:Blue Edge
415:Upgraded
405:BERP III:
362:composite
247:wave drag
137:does not
500:Archived
435:See also
423:BERP IV:
370:aerofoil
342:planform
335:aerofoil
284:and the
106:and the
194:scholar
158:removed
143:sources
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94:The
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